The currently defined American National Standards Institute (ANSI) standard T1. 601 governing 2B1Q modulation, two-wire, full-duplex data transfer with echo cancellation requires that ISDN basic rate digital subscriber lines (having a data rate of 144 kbps, with bidirectional data payload, plus overhead maintenance channels), must not exceed a two-wire loop loss of 42 dB at 40 KHz, for 1300 ohms, resistive. This requirement effectively limits the operational range of a conventional copper cable link (No. 26 (American Wire Gauge) AWG wire) driven by commercially available ISDN transceiver equipment to a distance on the order of 15 Kft.
The U.S. Pat. No. 5,598,413, to M. Sansom et al, (hereinafter referred to as the '413 patent), issued Jan. 28, 1997, entitled: "Four Wire, Half-Rate Architecture with Embedded Differential Delay Compensation for Extending Range of Basic Rate ISDN Communications," assigned to the assignee of the present application and the disclosure of which is herein incorporated successfully addresses this limited range problem by a one-half rate, four-wire, ISDN demultiplexing - multiplexing architecture, that uses an out-of-band maintenance channel for the transport of differential delay compensation information. Advantageously, this scheme is able to extend the normal range of ISDN basic rate digital subscriber lines to a distance on the order of 25 Kft, which is well beyond those currently possible using a repeaterless two-wire transmission path.
My co-pending U.S. Pat. application Ser. No. 08/744,975, filed Nov. 7, 1996, entitled: "Quarter-Rate 2B1Q Architecture with Embedded Differential Delay Compensation for Extending Range of DDS Communications," assigned to the assignee of the present application and the disclosure of which is herein incorporated (hereinafter referred to as the '975 application) solves a similar range limitation problem for four-wire DDS communications, by using a standard ISDN transceiver chip to demultiplex quarter-rate (2B1Q) ISDN channels for the transport of DDS data over a four-wire transmission path between an office channel unit data port (OCU DP) and a customer premises site. As in the scheme described in the '413 patent, by operating at a frequency that is one-quarter of the frequency associated with the ISDN transceivers, the reduced data rate of the four-wire system provides a trade-off against loop loss, increasing the distance over which DDS may be provided, without further stipulation or constraint upon the requirement of loops considered as DDS candidates in terms of loop loss and bridge tap.
Fortunately, because well over 95% of two-wire copper cable links currently installed in the United States do not exceed this (15 Kft) distance, the above described range limitations for the transport of ISDN and DDS channels are not yet a significant problem in most installations. However, because the physical layout of a typical copper link contains one or more bridge taps that are distributed along its length, and many of which are unterminated, the typical copper link is vulnerable to interference.